(CNN) -- FBI agents are trying to determine who stole 19 pieces of high-priced art, including an Andy Warhol silkscreen, from a Detroit business.

The art -- worth millions of dollars, according to CNN affiliate WDIV -- was taken between April 27 and April 29 from a business owned by an art collector in Detroit's Corktown neighborhood, the FBI said Tuesday. The agency named neither the business nor the owner.

The collection includes a 1960s silkscreen that Warhol used to make "Flowers" prints, according to the FBI. The other pieces of art, including paintings and drawings, were done by Larry Rivers, Francesco Clemente, Philip Taaffe, Joseph Beuys and Peter Schuyff.

Investigators suspect that the thief or thieves "may have already crossed state lines, if not left the country, in an effort to sell them," FBI spokesman Simon Shaykhet said.

"We're putting a message out to art dealers, pawn shop owners, and anyone dealing in art to be aware of it," Shaykhet said.

The art was neither locked up nor on display, the FBI said.

A $5,000 reward is being offered for the pieces' recovery. They have been entered into the FBI's national stolen art database.

Up to $6 billion worth of art is stolen each year, according to the FBI.

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